Consider deprecating LangChain vector search adapter

[abrookins]

The LangChain vector search adapter is limiting -- vector search implementations are not guaranteed to have any shared features, including metadata filtering. Probably better to build against our interface without an adapter, which still creates the opportunity to build alternative implementations.

[abrookins]

cc @nkanu17

[fcenedes]

linked with #70 ?

[abrookins]

@fcenedes Yes! 👍